This invention relates to a method for forming a composite lithographic plate, and, in particular, the invention relates to a method of making a composite lithographic plate which can be utilized with medium to long run printing applications. The invention also relates to a backing structure which can be utilized in forming composite lithographic plate.
In the printing industry, the useable life of a lithographic plate is defined by the number of clear impressions which can be obtained. The life span of a plate is normally divided into three categories-- short run, medium run or long run. The short run is usually defined as a plate which will give at least 5,000 to 20,000 clear impressions. The medium run plate will generally give up to about 100,000 clear impressions, while the long run plate will give in excess of 100,000 clear impressions. The short run plates are normally used on the simpler multi-copier machines such as duplicating machines since the short run plates are normally less substantial in strength than the medium or long run plates and their thickness generally has an upper limit of about five thousandths (0.005) of an inch. The short run plates are usually flexible enough to permit the surface to be prepared by a typewriter.
In view of the requirement for flexibility and in order to save metal, various composite lithographic plates have been developed for a short run application. Two of these lithographic plates are disclosed in the following patents--U.S. Pat. Nos. 2,344,510 and 3,229,628. Most of these plates include a metal layer, usually of aluminum; and, as can be seen in the above-cited patents, the metal layer usually has a thickness of between about five ten thousandths (0.0005) to two thousandths (0.002) of an inch. A paper layer of between twenty-five ten thousandths (0.0025) and ten thousandths (0.010) of an inch is provided as a backing member. The paper is usually waterproofed in order to withstand the various treatments the plate must be subjected to during processing to prepare its surface to receive the sensitizing coating.
The treating procedures usually include a degreasing step, a graining step-- either chemical or mechanical-- and, in some cases, an anodizing or solution bath which prepares the surface of the metal to receive a sensitizing coating. In the degreasing step, the plates are washed to remove any grease or foreign matter on the surface of the metal. The graining step which can be mechanical brushing or chemical etching provides a rough surface which aids in preparing the surface of the metal to receive the sensitizing coating. The anodizing of the metal as well as the solution-rinsing step further prepares the metal for the sensitizing coating. In each of these steps, water or some other liquid is utilized; thus, the plates are constantly being wetted.
As mentioned previously, the thinner plates are primarily used for short run applications and can not normally be used for the medium to long runs because they tend to stretch during use which causes distortions in the impressions produced. It can be easily understood to overcome this stretching problem in longer run printing applications, a thicker plate must be used. Surface quality of these thicker plates must be more uniformed than the short run plates and this is primarily due to economic reasons. For example, if a defect occurs in a short run plate making it useless, it is much less expensive than the thicker plates and it is discarded without much cost. The thin short run plates require more rolling to get them to an acceptable thickness and defects appear more readily. It is an advantage to having the thicker plates for long run use since the uniformity of the surface can be controlled and maintained more easily. Another characteristic of the long run lithographic plates is the surface flatness. The flatness is necessary to allow proper contact of the plate and the negative during the exposure process. If there are any buckles or waves in the metal plate, it will not lie flat on the vacuum frame of the photo-composing machine or the press cylinder. Therefore, it can be easily understood that a thicker gauge metal will retain its flatness more easily. Fortunately, flatness can be maintained much easier in the thicker gauge metals because the contact rolling which is required when manufacturing a thin gauge stock is not required for the thicker gauges.
In view of the above-defined differences between the characteristics of short and medium to long run plates, it can be easily understood that a composite lithographic plate previously known in the art which is used for a short-run printing application cannot be utilized for the medium to long-run jobs. Furthermore, the industry has not turned to laminated composite plates with thicker gauges primarily because no suitable structure has been found which can withstand the processing (degreasing, graining, anodizing, etc.) which is required without the delamination of the structure. Delamination is not a problem with the thin gauge composite lithographic plate because the thinner layers do not absorb the liquid as readily when the composite plates are being processed. When constructing a thick composite plate for use in medium to long run printing applications, the backing member for the metal must necessarily be thicker than the backing members previously known in the art in order to compensate for the much greater reduction in metal thickness. This increased thickness of the backing member makes it much easier for the liquid used in the processing to be absorbed therein, thus, the delaminating problem is more acute. Another factor which must be taken into consideration when making composite plates for longer run applications is the use of press packing paper shims to properly space and align the plates on the presses. If a composite plate were used, one advantage of such a plate would be to eliminate or minimize the requirement for packing material since the backing member could be any thickness desired. Increasing the thickness of the backing layers to eliminate the packing material would also increase the area absorbing the processing liquids and, thus, add to the delamination problem.